Blasting caps containing only secondary explosive



M. REYNE Oct. 19, 1965 BLASTING CAPS CONTAINING ONLY SECONDARY EXPLOSIVEFiled NOV. 21, 1962 COMEUJ 770/V Mau/vc e Rey/7e INVENTOR United StatesPatent 3,212,439 BLASTING CAPS CONTAINING ONLY SECONDARY EXPLOSIVEMaurice Reyne, Bonrg-la-Reine, France, assignor to Societe deProspection Eiectrique Schlumbcrger, S.A., Paris, France, a corporationof France Filed Nov. 21, 1962, Ser. No. 239,239

Claims priority, application France, Nov. 24, 1961,

880,063 4 Claims. (Cl. 102-28) The invention relates to blasting capsand, more particularly, to blasting caps for detonating shaped chargesin a well bore.

It is known that conventional blasting caps, which contain a certainamount of explosive of the primary type such as lead-azide, aredangerous because of their sensitivity to shock forces which may causedetonation. Blasting caps are typically used to detonate a lesssensitive secondary explosive such as a blasting cord comprised ofcyclonite. While secondary explosives are just as powerful as theprim-aw explosives, fundamentally, they differ in being practicallyinsensitive to shock. Therefore, according to regulations applicable toair transport, for example, detonators or blasting caps containing aprimary explosive cannot be transported by airplane while secondaryexplosives may be transported by airplane. Since oil well servicesrequire equipment to be shifted over great distances, it is desirable tohave a blasting cap which is relatively insensitive to shock and isabsolutely safe and appropriate for air-lift. This safety will also beappreciated by technicians at the time of use of the blasting cap.

An object of the invention is to provide a blasting cap which containsonly an explosive of the secondary type which is not particularlysensitive to shock.

According to another feature of the invention, the explosive is in theform of grains which are sized between three hundred and five hundredmicrons.

A detonator according to the invention comprises, within a closedcontainer tube of appropriate thickness, a certain amount of secondaryexplosive and an igniter made of an electrical resistor and aninflammable explosive, and is characterized by the fact that the portionof said tube containing the secondary explosive has a bore of eightmillimeters in diameter and an inner length of thirty-seven millimeters,these dimensions being determined with an accuracy within ten percent,and by the fact that said explosive is of the secondary type, with areaction zone" whose length is less than three millimeters.

Owing to this arrangement, a confined enclosure of minimum size providesconditions under which a flame ignites a secondary explosive whichignition is transformed into a detonation. One thus achieves a safetydetonator which is relatively insensitive to shock.

Secondary explosives are classified according to their sensitiueness,which is an essentially qualitative datum, yet also, and in the sameorder, according to the reciprocal of the length of their reaction zone.Reference may be made to The Science of High Explosive, by M. A. Cook,1958, Reinhold Publishing Corporation, and in particular pages 123 and124 and Table 7.3 on page 147 for further details. However, according tothis study, the reaction zone of an explosive is the thickness in whichthe chemical reaction develops at the maximum velocity for the explosiveconsidered. The lengths of reaction zones of various explosives areknown; they were measured by means of ultra-fast photography andelectronics. Thus, one knows that hexogen exhibits a reaction zone ofone millimeter, Whereas coarse grained TNT exhibits a reaction zone offifteen millimeters.

Characteristics and advantages of the invention shall a depth rangingbetween 5 to millimeters.

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be set forth, again by the description to follow supplied merely as anexample, reference being taken to drawings attached where:

FIG. 1 illustrates a cross-section of a detonator according to theinvention; and

FIG. 2 illustrates a cross-section of a modified detonator according tothe invention.

As shown in FIG. 1, a steel tube 10 is closed at one end 12 by a thinmetal lid 14 and at its other end 16 by a conical metal liner 18. End 12is machined down at 22 and is received in a hollow metal adapter 24Whose upper portion is provided with a counter bore 26 into which isintroduced the insulating wlad 28 of an igniter of the type described inthe United States Patent No. 2,681,701, issued to M. Schlumberger. Abore 30 in adapter 24 is traversed by the igniter needle 32 filled withblack powder. An electrical resistor 34 is connected across theinsulating wad 28 to a conductor 36, whereas its other end is linked at38 to the outer conducting wall of needle 32. Conductor 37 attached onthe periphery of adapter 24, establishes, by means of lid 14 being incontact with the metal wall of needle 32, the return circuit for theheater current of resistor 34.

The lower end 40 of needle 32 goes through lid 14 to The interior 42 oftube 10 is filled with pentrite and compressed in the tube 10 by a ramunder a low Weight (150 kilograms) inserted into end 16. Dimensions ofthe ex plosive are determined by the use of two sieves whose aperturesare three hundred and five hundred microns. The lower portion 16 of tube10 is machined down at 44 where a cylindrical plug 46 is attached. Anylon screw 48 extends axially into a recess 47 provided in the plug 46.An opening 50 is drilled transversely through plug 46 and a blastingcord 52 can be introduced into this opening and fixed therein betweenthe base of plug 46 and screw 48.

If a voltage of a suitable value is applied across the extremities ofconductors 36 and 37, resistor 34 heats up to a point where the blackpowder ignites and a flame is introduced rather deeply into the pentrite(which is in a pulverulent condition adjacent to the needle-igniter 32).This flame, in the first place, causes the pentrite to be set afire.But, since the walls of tube 10 are of a considerable thickness, saythnee millimeters, a suitable confinement is secured and as thecombustion progresses within tube 10, it progressively develops into adeflagration then into a detonation which detonates the blasting cord52.

Systematic measurements and testing were performed on a detonator asdescribed in FIG. 1. Using various especially sensitive secondaryexplosives, that is explosives exhibiting reaction zone lengths helowthree millimeters, such as pentrite, hexogen and tetryl, the innerdiameter of tubes 10 was changed from one test to another. The criticalvalue was determined experimentally from these tests that below eightmillimeters, the detonation phenomenon developed poorly. Thereafter, thelength of this same tube 10 was reduced progressively until detonationno longer occurred. The critical length was thus determined, which isapproximately equal to thirtyseven millimeters. Of course, these figuresrepresent merely an order of magnitude with an accuracy of some tenpercent, and one would still obtain a suitable operation by reducingvery slightly the bore of the tube While the length is being increased.In practice, only smallsized detonators offer some interest for oilfield work.

Attempts were also made to use medium sensitivity secondary explosives,that is, explosives exhibiting reaction zone lengths exceeding thneemillimeters. Thus,

after having obtained satisfactory results with hexogen,

pentrite and tetryl, unfavorable results were yielded with hexyl, toliteand me linite, and even while using tube diameters notably greater thanthe eight millimeters mentioned above.

The grid sizes and the compression of various explosives withheld fortheir good working were then changed. Using two sieves exhibitingapertures either greater than three hundred microns or smaller than onehundred and fifty microns in order to select the particles of explosiveaccording to their size, high failure rate re sulted, whereassatisfactory detonation is obtained if two sieves with apertures betweenthree hundred and five hundred microns are used. Similarly, increasingthe compression rate of the explosive beyond one hundred and fiftykilograms prevents the flame from penetrating appropriately into themass, which in turn results in failures.

In the course of the operation of the detonator of FIG. 1, the conicalliner 18 subsides under the effect of the detonation of pentrite fillingvolume 42, and thus improving the detonator performances still further,contributes to the detonation of the blasting cord 52. Such a shockproofdetonator may be successfully employed in retrievable carrierperforators, but only in the case of shallow wells since pentrite doesnot perform properly at temperatures above 120 C. In order to obtain ablasting cap for use in greater depths, one would resort to hexogenwhich performs properly at 180 C.

Adapter 24 can also be provided with an aperture 25 to establish anopening between duct 30 of the detonator and the inner volume of thecarrier perforator. Hence, if

.the carrier leaks fluid under pressure within the borehole,

such fluid would gain admittance to the blasting cap to desensitize it.

Another embodiment of a detonator according to the invention is shown inFIG. 2.

According to FIG. 2, a steel tube 54 with walls at least threemillimeters thick provides a chamber 42 which is filled with hexogenunder conditions of grid sizes and compression as described above. Athin-walled case 56 filled with a thermite mixture 58 such as a weaklycompressed homogeneous mixture of aluminum and barium peroxide powder inroughly stoichiometric proportions (Al: 20% and BaO: 80%). An electricalresistor 60 is connected by two conductors 62 and 64 through glass beadseals 66 and 68 to an outer power supply. At the end of the detonator,both conductors 62 and 64 are mechanically secured by an insulating wad69. Case 56 is attached to a cover 70 which rests on a shoulder 72 pro-,vided inside the tube 54. Cover 70 is fastened by welding at 74attaching the thinned down inner wall 76 of said tube 54. At the lowerportion of chamber 42 is an amount of hexogen explosive, the chamberterminating with a bottom wall 78 approximately one millimeter thick.

-On the other side of wall 78, tube 54 is terminated by a .82, the endof the blasting cord 84 in contact with the -wall 78.

The thermite powder contained in case 56 offers particularly remarkablesafety features and the cover 70 welded totube 54 and glass beads 66 and68 provide a fluid-tight seal which also will not leak gas. Under theseconditions, the combination in leak-proof receptacle of a case 56containing thermite powder 58 and of a tube 42 of a suitable length anddiameter filled with hexogen, achieves a safety detonator which isindilfcrent to shock and, moreover, indifferent to static charges, tospurious currents and to hot gases under pressure.

The invention is, of course, not limited to the embodiments describedabove which have merely been supplied for information purposes, butwhich, on the contrary, within the scope of the invention, may form thesubject of various alternatives.

Thus, either with an ignition needle or with a thermite booster charge,one may use tetryl instead of hexogen or pentrite in order to achievedetonators according to the invention. But like pentrite, tetrylprovides for detonators to be used at temperatures up to less than C.corresponding to wells of small depths.

One can also envisage different ways to establish the contact betweenthe secondary explosive, mass and the blasting cord. Thus, for instance,in order to bring about the detonation of explosive charges intended toclear boring tubes, the ends of several blasting cords forming saidcharges are paralleled around the body of a detonator according to theinvention.

Furthermore, one can, of course, use other means as setting or weldingin order to fasten among them the various elements making up a detonatoraccording to the invention.

I claim:

1. A blasting cap for use with oil well perforators consisting of: ahollow steel tube with a wall thickness of at least three millimeters,an inner diameter dimension of eight millimeters and a length dimensionof thirty-seven millimeters, said diameter and length dimensions beingheld to an accuracy of ten percent, said tube being substantially filledwith a secondary explosive having a reaction zone of less than threemillimeters, a closure for one end of said tube including an electricaligniter and a closure for the other end of said tube including a thinwall portion for transmitting a detonation wave therefrom.

2. The apparatus of claim 1 wherein said igniter includes a needlecontaining black powder, the tip of said needle extending into saidexplosive for a depth ranging between 5 and 10 millimeters.

3. The apparatus of claim 1 wherein said explosive has a grain sizebetween three hundred and five hundred microns.

4. The apparatus of claim 1 wherein said explosive is selected from agroup consisting of hexogen, pentrite and tetryl.

I References Cited by the Examiner UNITED STATES PATENTS 2,681,701 6/54Schlumberger l0228 XR 2,891,477 6/59 Swanson l0228 2,981,186 4/ 61Stresau l0228 SAMUEL FEINBERG, Primary Examiner.

BENJAMIN A. BORCHELT, Examiner,

1. A BLASTING CAP FOR USE WITH OIL WELL PERFORATORS CONSISTINF OF: AHOLLOW STEEL TUBE WITH A WALL THICKNESS OF AT LEAST THRE MILLIMETERS, ANINNER DIAMETER DIMENSION OF EIGHT MILLIMETERS AND A LENGTH DIMENSION OFTHRITY-SEVEN MILLIMETERS, SAID DIAMETER AND LENGTH DIMENSIONS BEING HELDTO AN ACCUARACY OF TEN PERCENT, SAID TUBE BEING SUBSTANTIALLY FILLEDWITH A SECONDARY EXPLOSIVE HAVING A REACTION ZONE OF LESS THAN THREEMILLIMETERS A CLOSURE FOR ONE END OF SAID TUBE INCLUDING AN ELECTRICALIGNITER AND A CLOSURE FOR THE OTHER END OF SAID TUBE INCLUDING A THINWALL PORTION FOR TRANSMITTING A DETONATION WAVE THEREFROM.